Development of ground-based search-coil magnetometer systems in the polar regions and studies of ULF Pc 1--2 wave propagation in the ionospheric waveguide.

摘要

Search-coil magnetometers, which measure time-varying magnetic flux density (dB/dt) and its direction, have been developed for the observations of geomagnetic pulsations in the ultra low frequency (ULF) range (a few mHz to a few Hz). The design, fabrication, and test/calibration have been performed to detect very weak geomagnetic pulsations with approximately a few pT resolution over the frequency range 0 --- 2.5 Hz and 100 musec timing accuracy, given a system gain of 4.43 V/(nT·Hz) and 12-bit analog-to-digital converter (ADC) with GPS time stamps. These instruments are deployed in the Polar regions forming high latitude networks and conjugate measurement points between the northern and the southern hemispheres. In addition to the development and installation of the magnetometers, this thesis describes analysis of the data from the magnetometer systems, mainly focusing on ULF wave propagation in the ionospheric waveguide (duct) centered around the electron density maximum near the F2 ionization peak. The Antarctic magnetometer array observes well-defined, band-limited ULF Pc 1--2 waves with poleward spectral power attenuation over a very extensive latitudinal coverage from geomagnetic latitudes of --62° to --87° (over the distance of 2920 km). This is a clear indication of the propagation of the electromagnetic ion cyclotron (EMIC) waves in the ionospheric waveguide. This study focuses on the ducting events by comparing spectral power attenuation factors and polarization patterns. A statistical survey of the events reveals that the attenuation factors are between &sim10 to 14 dB/1000 km and the polarization sense changes as the waves are ducted poleward from the low latitude regions. For a detailed event study, a CHAMP satellite conjunction is presented. During the overflight, a transverse and linearly polarized Pc 1 ULF wave was also found over a limited latitudinal extent (--53° to --61° ILAT), which supports the idea that EMIC waves are injected at low latitudes and ducted in the ionosphere. The results show the observations of ducted waves over such an unprecedented latitudinal extent, which have rarely been measured before, and thus provide very important information about ionospheric wave ducting characteristics.